1. Technical Field
The present invention relates to a hot-press deep-drawing forming technology that allows obtaining a high-strength product through deep-draw forming of a steel plate, in a state where the steel plate is heated at or above a transformation temperature, followed by die quenching of the formed steel plate.
2. Related Art
A steel plate, as a workpiece, is worked by draw-forming in order to form a product, having a three-dimensional shape through press-working of the steel plate. Products that are worked by forming include products that have: a main body having a U-shaped cross-sectional shape and made up of a front wall and side walls that are contiguous with the front wall; and a flange that is integral with the main body.
A press die for drawing a product having such a three-dimensional shape has: a die provided with a forming surface corresponding to the outer surface of the main body; a punch provided with a forming surface corresponding to the inner face of the product; and a blank holder that presses the edges of the workpiece, i.e. the steel plate, to suppress thereby the occurrence of wrinkles in the steel plate. Japanese Unexamined Patent Application Publication (JP-A) No. H2-205210 discloses a press die having a movable punch for intermediate drawing, and having upper and lower blank holders, wherein drawing of the central portion of the workpiece, and reverse-drawing of the outside of the workpiece, are performed through a single working operation of a press ram, JP-A No. H8-90094 discloses a drawn product forming apparatus in which a stepped drawn product is formed over one stroke. This apparatus has a lower die provided with a vertically movable blank holder, a punch that: can advance and retreat inward of the upper blank holder, and a vertically movable lower blank holder that clamps a thin plate together with the upper blank holder,
JP-A No. 2011-50972 discloses a hot-press forming method. In this forming method, one stroke from forming start to forming termination is set to obey different forming conditions in initial-stage forming, middle-stage forming and final-stage forming, using a hot-press die that has a punch, a die and a die cushion, so that, as a result, wrinkles that occur halfway during forming are removed at the final-stage forming. JP-A No. 2010-69535 discloses a forming method that involves press quenching using a press apparatus having a lower forming tool, a vertically movable upper forming tool, and a blank holder, wherein a partially non-quenched region is formed at the edge region of a press-quenched steel plate.
JP-A No. 2005-297042 discloses a deep drawing method in hot forming. In this deep drawing method, a steel plate is heated at or below the melting point, and is formed at or above the transformation temperature, and the clearance between a blank holder surface and a die face is set to be equal to or smaller than the clearance of a gap, greater than, the thickness of the steel plate, and for which there occur wrinkles derived from contact of a same surface of the steel plate.
The above publications JP-A No. 2011-50972 and JP-A No. 2010-69535 both involve a method of flattening wrinkles at the final stage of forming, while allowing for the occurrence of wrinkles daring the forming process. However, there are parts difficult to form that give rise to problems such as excessive wrinkling during the forming process, with unacceptable wrinkles, or occurrence of wrinkles that cannot be fully flattened at the final stage of forming, with non-uniform contact between a die punch and a die. In cold working, by contrast, step drawing is performed in some instances in addition to that by a blank holder mechanism (for example, JP-A No. H2-205210). In hot press dies, however, there are sites where the workpiece continues to contact the die during the step drawing or the step performed by the blank holder mechanism. This may lead to unintended ongoing removal of heat from the workpiece, that gives rise to impaired ductility of the workpiece and to forming defects such as breakage in the formed product.